Combine flour, cornmeal, sugar, baking powder, and soda in a large bowl; cut in butter with a pastry blender until mixture resembles coarse meal. Add buttermilk; stir just until dry ingredients are moistened. On a lightly floured surface, knead dough 5 or 6 times. Roll dough to 1/2" thickness; cut with a 2-1/2-inch biscuit cutter. Place on a baking sheet coated with cooking spray. Brush tops with egg white. Bake at 450�F for 12 minutes or until golden.